This invention is directed to an industrial wastewater treatment method wherein a neutralizing agent is adsorbed on the surfaces of a carrier and the carrier-neutralizing agent combination is added to the wastewater to adjust the pH of the wastewater to a desired range whereby a substantial portion of heavy metals dissolved in the wastewater is precipitated to form a slurry which if allowed to settle will form a sludge containing a relatively high percentage of solids. The sludge is easily separated from the water. In the event that insoluble calcium salts are also precipitated, such salts are precipitated on the surfaces of the particles of solids in the slurry. Wastewater so treated is purified sufficiently to allow its reuse in industrial processes or discharge to the environment, for example into surface waters.
The discharge of industrial wastewater containing dissolved concentrations of heavy metals, for example iron, manganese, copper, tin, lead, nickel, mercury, zinc, cadmium, and the like to the environment is frequently prohibited because such heavy metals may be toxic to aquatic life and may also render the surface waters unfit for human consumption. Reuse of such contaminated wastewater in industrial processes is also limited because, as the concentration of heavy metals increases during reuse, such concentrations may become sufficient to poison the processes in which they are reused. It is therefore frequently desirable for wastewater be treated to either remove substantially all heavy metals or to reduce the amounts of the dissolved heavy metals to levels at which the water is considered safe for both aquatic and human life prior to disposal in surface waters and which are considered to be satisfactory for reuse in industrial processes.
Many processes for the removal of dissolved heavy metals from industrial wastewater have been proposed. Several processes advocate the addition of iron and alkali metal or alkaline earth metal hydroxides to the wastewater to adjust the pH of the wastewater whereby the heavy metals will be precipitated from the wastewater. Generally a pH within a range between about 8 and 11 is used. The mixture thus formed is heated to a temperature between 40.degree. and 100.degree. C. so that the heavy metals will precipitate. An aerating environment may also be required to precipitate the heavy metals as ferrites. Since the solubility of the various heavy metals varies with the pH of the solution in which they are dissolved, a portion of the heavy metals which are precipitated at pH levels which are relatively neutral, or about pH 8, generally will be resolved when more basic pH levels, i.e. pH 9 to 12, are achieved to precipitate other heavy metals. As a result, the wastewater frequently becomes recontaminated with the heavy metals which are resolved. To treat a wastewater which contains a wide range of heavy metals, it is first necessary to adjust the pH level of the wastewater to 8 to 9 to precipitate at least some of the heavy metals. To prevent resolvation of the precipitated heavy metals, it is then necessary to separate the precipitated solids by some method, such as filtration, prior to again adjusting the pH level of the water to remove more of the dissolved heavy metals. The pH adjustment process must be repeated at various pH levels to be successful in the removal of a sufficient amount of the heavy metals to attain the desired low levels. Such a multistep or multistage process is time consuming and requires extra equipment for transport and means for removing the thin, gelatinous-like precipitated solids, and retreating the wastewater. As a result, such multistep processes are expensive. The settled precipitates produced in the processes are low in solids content, generally between 0.5 to 1 weight percent solids and occasionally only as much as 5 weight percent solids. The gelatinous-like precipitates are difficult to separate from the water. Separation requires large expensive thickeners, flocculators, filters and the like to separate the gelatinous-like solids from the water. None of the processes described above, furthermore, address the problem caused by the formation of insoluble calcium salts. A portion of the precipitated calcium salts are not removed with the precipitated heavy meatals, but are deposited on the surfaces of the equipment and piping used to treat or transport the wastewater. Layers of the salts accumulate and eventually clog the equipment. It is therefore necessary to periodically shut down the equipment and remove the deposited salts thereby adding to the costs of the process.
Other processes used to treat acidic industrial wastewater, for example acid mine drainage, waste pickle liquor and the like which contain ferrous and/or ferric ions, require the use of a neutralizing agent, for example limestone, to neutralize the acidity of the water and to adjust the pH of the wastewater from between about 2 and 5 to about 7.0 and 8.4. In at least one of these processes sludge relatively high in solids content is produced. The process requires that a specified amount of sludge containing a specified amount of solids must be recycled in the process to precipitate a sufficient quantity of solids to maintain the solids content of the sludge. In the processes a large portion of iron and a small portion of some heavy metals, for example, manganese or zinc, are precipitated and removed at a pH of about 8 to 8.6. However, the processes do not at one time solve the problem of removal of all the toxic heavy metals within a pH range of 8.4 to 10.6. Nor do these processes address the problems associated with the precipitation of insoluble calcium salts which often occurs when a calcerous material is used as a neutralizing agent.
None of the prior art processes mentioned above solve the problem of removing heavy metals from wastewaters by mixing a neutralizing agent adsorbed on a carrier with industrial wastewater containing dissolved heavy metals to effectively adjust the pH of the wastewater to a desired range whereby a substantial portion of the heavy metals dissolved in the wastewater will be reduced to acceptably low levels by precipitating them in the form of a slurry which when allowed to settle will form a high solids sludge.
It is the object of this invention to provide an industrial wastewater treatment method to remove dissolved heavy metals from wastewater wherein the wastewater is mixed with a neutralizing agent adsorbed on the surfaces of a carrier to adjust the pH of the wastewater to a desired pH range at which a substantial portion, if not all, of the dissolved heavy metals is precipitated as a slurry which, if allowed to settle, will form a sludge containing a high percentage of solids with little if any resolvation of the heavy metals.
It is another object of this invention to provide an industrial wastewater treatment method in which a portion of the sludge formed in the method is recycled in the system.
It is also an object of this invention to provide an industrial wastewater treatment method in which a flocculant is added to the treated wastewaters to aid in settling the slurry formed during the neutralizing step.
It is a further object of this invention to provide an industrial wastewater treatment method in which insoluble calcium salts are precipitated on the slurry solids of heavy metals and the calcium salts are separated from the water as part of the sludge.